I'm trying to convert an XYZ file from WGS84 (lat/long) to UTM ED50 zone 34. QGIS can load the file as a raster, but I can't see a way of changing the coordinate system or exporting the results as an XYZ file.

Have you seen this question? It has some useful info and links that should point you in the right direction, but may be slightly different depending on your version of QGIS. Unfortunately, I don't have a install of QGIS handy so I can't answer you definitively.
– Evil GeniusSep 23 '14 at 15:18